1. Related Field
The present invention relates to optical presentation devices for vehicles. More particularly it relates to devices for combining an observer's view in front of a vehicle with a synthetic image by superimposing the latter on the former, so called head-up displays. Even more particularly it relates to head-up displays suitable for use together with night vision goggles.
2. Description of Related Art
In fast moving vehicles, such as aircraft, it may constitute a risk during flight of the aircraft, each time a pilot of the aircraft has to split his visual attention between what is happening in front of the vehicle and what is happening inside the vehicle, in particular on a dashboard showing flight information. Pilots of aeroplanes and helicopters are therefore often helped by a device called head-up display.
In a vehicle equipped with a head up display system, an observer, which usually is the pilot, observes the outside world through a windscreen of the vehicle, and a semi-transparent combiner mirror of the head-up display arranged in his forward line of sight such that a light ray from a distant object passes through the windscreen and then through the semi-transparent combiner mirror before it reaches an eye of the observer. Image information, such as flight information symbols of an image source are reflected to the semi-transparent combiner mirror of the head up display, and appear for the observer to be superimposed on the perceived image of the outside world. This has the advantage that the observer does not have to shift his eyesight for gathering flight information. He also avoids having to refocus eyesight, since optical elements usually are arranged to provide a so called collimated image.
Head-up display systems are known for use in e.g. military fighter aircraft. These display systems present flight information using a certain colour, and the combiner mirror is provided with certain layers of optical coating to reflect light of that certain colour better than light of other colours. This also means that light of other colours are not reflected as much and therefore are transmitted better through the semi-transparent combiner mirror.
A different aspect of flying an aircraft is that during night vision conditions it is very hard to precisely manoeuvre the aircraft in relation to the environment, since the pilot has to rely on instrument readings instead of eyesight of the environment. It is therefore an advantage to use night vision goggles, i.e., goggles that amplify even very faint light captured by them such that it is possible for a pilot wearing the goggles to observe the ground and the environment in front of and around the vehicle in spite of the darkness.
A further aspect of flying such a vehicle is that it would be advantageous if the pilot, equipped with night vision goggles, also could take advantage of the benefits of a head up display. However, it is not straightforward to take a head-up display system and fit it into a vehicle whose pilot already uses a pair of night vision goggles, and expect it to work well. This have been realised by the inventors and will be further discussed below.
However, a main object of the present invention is to provide a head-up display system that can be fitted into an aircraft, and whose pilot already is equipped with a pair of night vision goggles.